A power conditioner for photovoltaic power generation which coordinates a solar battery with an AC power supply system has a function of converting DC power generated by the solar battery into AC power, and a function of coordinating the solar battery with the AC power supply system. The AC power sent to the AC power supply system is consumed by a load connected to the AC power supply system. For example, a system coordination inverter as a power conditioner for photovoltaic power generation converts DC power inputted from a DC power supply (solar battery) into AC power via a converter circuit and an inverter circuit in which the inputs and outputs are not isolated from each other, and outputs the AC power to an AC power supply system that is grounded (see, for example, Patent Literature 1).
In the above configuration, the solar battery and the AC power supply system are connected in a non-isolated state. In addition, the neutral point of the AC power supply system is grounded. Therefore, the solar battery needs to be isolated from the ground (earth), and as a result, a floating capacitance Cs occurs between the solar battery and the ground. A neutral point voltage of the AC power supply system is almost equal to a DC neutral point voltage of the inverter circuit. Here, if the voltage of a DC capacitor connected to the DC side of the inverter circuit is defined as 2 E (V), the voltage of the floating capacitance Cs (the voltage of a negative electrode N as seen from the ground) is −E (V). That is, the negative electrode N of the solar battery is negatively biased.
Solar batteries are classified into a crystal type and a thin-film type. It has been reported that if a solar battery of thin-film type is used while its negative electrode N is negatively biased, the deterioration of the solar battery is accelerated (see, for example, Non-Patent Literature 1). Therefore, when a solar battery of thin-film type is used, in general, the negative electrode N is grounded. However, the grounding method for the negative electrode N is not applicable to non-isolated systems.
In addition, the following technique is known. In order to suppress the deterioration of a solar battery, an isolating transformer is provided to a power conversion section for converting DC power of the solar battery into AC power so that the solar battery is coordinated with an AC power supply system via the isolating transformer, and a voltage conversion section is provided to shift the voltage against the ground of each of the positive electrode and the negative electrode of the solar battery, to a value larger than 0 (V) (see, for example, Patent Literature 2).